The present invention is in the field of coating processes and in particular it is directed to a process for coating granulated material obtained in a damp state.
A variety of coating methods and processes are known for coating of granulated or particulated material. Typically, such coating methods are intended for encapsulating the granulated material in order to improve its resistance to humidity and to oxygen, so as to maintain the properties of the material.
Accordingly, the nature of such coatings is that they are liquid repelling and impervious to gasses.
Typically, coatings of this nature are applied by precoating the granulated material with an adhesive agent and then applying the coating substances. In other cases, a liquid solution is applied over the granulated material and is then allowed to dry.
Methods of coating are disclosed, for example, in the following U.S. Pat. Nos. 3,640,787, 3,989,852, 5,308,396 and 5,935,626.
It is noted, however, that in accordance with the present invention the coating material is required to be liquid-permeable so as to allow absorption of liquid by the granulated material.
According to a first aspect of the present invention there is provided a process for multi-layer coating of bulk granulated material, the process comprising the following steps:
(a) obtaining a bulk of granulated material in a damp state;
(b) premixing the granulated material with a first coating agent provided in powdered form;
(c) introducing the premixed material of step (b) into an inlet-end of a spiral winding path within a rotating drum;
(d) rotating the drum at a speed so as to advance the material along the winding path towards an outlet-end thereof;
(e) introducing at least a second coating agent into the drum; and
(f) drying the coated material.
In order to obtain best results, the second agent is applied over the material from above in either powdered form or as a fine spray. However, in a preferred embodiment, no liquids are added during the coating process.
According to a preferred embodiment of the present invention, the premixed material of step (b) is introduced into the drum at a continuous rate while the drum rotates at a fixed speed.
By a specific design, the winding path is defined by spiral walls radially extending from a wall of the drum, which walls are preferably coated with a material preventing adherence of the coated material.
Typically, step (b) is carried out by a screw-type mixing conveyor also referred to as xe2x80x9cribbon-screwxe2x80x9d or xe2x80x9cribbon-augerxe2x80x9d conveyor.
An important character of the invention is that essentially no liquid is added during the coating process wherein the liquid required for adhering the powdered material is extracted from the granulated material.
Preferably, the bulk granulated material is an agglomerated material having a substantially high absorbing capacity. However, the invention is not restricted to agglomerated material.
According to a second aspect of the present invention there is provided a dryer suitable for use in a process according to the present invention, the dryer comprising a housing, and a woven-wire conveyor belt extending within the housing between a loading end and an unloading end of the conveyor belt. The conveyor belt passes through a zone of air at ambient temperature adjacent the loading end; a zone of warm air (about 50-70xc2x0 C.) downstream of said ambient air zone and a hot air zone (about 100-150xc2x0 C.) adjacent the unloading end of the conveyor belt; and a pneumatic disperser for dispersing the granulated material above at least a portion of the conveyor belt.
In one specific and preferred embodiment, intermediate the unloading end and the hot air blower, the conveyor belt extends through a second zone with air at ambient temperature. Preferably, there is provided at least one blower for propelling air through the different zones and through a heat exchanging device. The heat exchanging device is preferably a burner.
In another preferred embodiment, curtain walls are provided between corresponding zones of ambient temperature and the warm and hot zones, respectively, for separating zones at ambient temperature from heated zones.
There is also provided a method for using a dryer of the above-mentioned type wherein the method comprises the following steps:
(a) Loading the granulated material on a metal woven conveyor belt;
(b) Exposing the granulated material to air at ambient temperature;
(c) Exposing the granulated material to warm air;
(d) Exposing the granulated material to hot air;
(e) Unloading the granulated material from the conveyor belt.
Typically, prior to step (e) the granulated material is exposed to air at ambient temperature.
At least during step (c) the granulated material is dispersed above the conveyor.